A typical optical receiver (Rx) includes at least one photodiode that detects an optical signal and converts it into an electrical current signal and at least one transimpedance amplifier (TIA) that converts the electrical current signal into an electrical voltage signal. The photodetector, which is typically a P-intrinsic-N (PIN) photodiode, produces an electrical current signal in response to light detected by the photodetector. The TIA converts this electrical current signal into an output voltage signal having some gain, commonly referred to as transimpedance gain. This output voltage signal is further processed by other circuitry of the optical Rx (e.g., a limiting amplifier (LA), clock and data recovery (CDR) circuitry, etc.).
The TIA circuit typically includes several control loops for improving performance, such as a direct current (DC) offset cancellation loop, an automatic variable gain amplifier (VGA) stage loop, and a TIA feedback impedance adjustment loop. Each of these loops utilizes analog components such as operational amplifiers (Op Amps), capacitors and resistors to implement the analog functionality needed. Such components are complex and require a large area on an integrated circuit (IC) chip in order to implement them with accuracy. In high-speed TIA circuits, such analog control circuitry can occupy the majority of the IC chip area, which increases the cost of the IC chip solution and leads to parasitic capacitances that degrade RF performance and/or increase power consumption. In addition, using analog methods to tune the feedback resistance of the TIA feedback impedance adjustment loop is not precise and suffers from limitations due to the precision and order of the adjustment function, the number of elements that can be controlled, etc.
A need exists for a TIA circuit having control loops that have greater precision and improved performance, that can be implemented in a smaller area on the IC chip, and that can be manufactured with improved IC chip yield and at reduced IC chip costs.